Shuttle physics
Posted by aogMonday, 03 February 2003 at 08:06 TrackBack Ping URL
I was listening to coverage of the loss of Columbia on NPR when they talked about the heat of re-entry in terms of friction with the atmosphere. This is a common misconception. One can see part of the reason if one considers that commerical jetliners, which fly at .8 or .9 mach don't have heating problems but planes that fly less than twice as fast (less than mach 2) do. The actual source of heat is compression. The ideal gas law, pV=NRT, states (among other things) that if you have a constant volume (V) and you increase the pressure (p) the temperature (T) increases. For commerical airliners, the pressure at the leading edge doesn't increase very much because it can dissipate into the general atmosphere. The speed of pressure waves is by definition the speed of sound so above mach 1 the pressure can't dissipate. This means that the air piles up in a layer in front of the leading edges. The size of this layer is roughly constant but it becomes much denser and as a consequence much hotter. It is this compression that generates the plasma of re-entry, not friction. One way to see this is to imagine how friction could heat the air in front of the shuttle.
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pj Monday, 03 February 2003 at 11:25

Good observation . . . But the fundamental point is that the potential energy between orbit and ground has to be converted to something other than kinetic energy in the shuttle, and dissipated. It’s going to get converted to heat somewhere. If you can’t dissipate the heat by sound waves, you can dissipate it by radiation.

Annoying Old Guy Monday, 03 February 2003 at 18:09

Well, once we build the space elevator we’ll just convert to electricity to boost the yield from the solar power satellites :-)

pj Tuesday, 04 February 2003 at 11:26

Yes - or help lift the next elevator car into orbit!

End of Discussion